Generally, a lathe is configured such that a cutting insert comes in contact with a rotating workpiece and cuts the workpiece while feeding along a rotational axis. In such a case, a safe fixation of the cutting insert to the tool holder is an important factor for achieving a desired cutting performance.
In the cutting insert (10) of the prior art, the top surface had an elongated concave shape in the left and right directions as shown in FIG. 1. The clamping arm of the tool holder presses the concave portion on the top surface of the cutting insert (10) in the downward direction to firmly secure the cutting insert. However, in such a case, only the frictional force between the clamping arm and the top surface of the cutting insert constrains the cutting insert in the horizontal direction. Thus, the cutting insert (10) may be pulled out of the tool holder.
In order to solve such a problem, an angled surface (22) was formed on the top surface of the cutting insert (20) in the prior art as shown in FIGS. 2 and 3, while the clamping arm (30) of the tool holder formed a nose (32) for mating with the angled surface (22). Accordingly, when the clamping arm (30) presses the cutting insert (20), the pressing force may be directed in the angled direction toward the tool holder, thereby preventing the cutting insert (20) from being pulled out of the tool holder (30, 40). However, in such a case, since the contact area between the clamping arm (30) and the angled surface (22) becomes extremely narrowed, the clamping force cannot be sufficient to fix the cutting insert (20).
In order to solve the above problem, the prior art employed a structure wherein the cutting insert can be simultaneously pressed both in the vertically downward direction and the angled direction, as disclosed in U.S. Pat. No. 6,176,649 (see FIG. 4). However, according to the structure shown in FIG. 4, the front portion of the clamping arm must have a nose in order to press the front portion of the cutting insert in the angled direction. The clamping arm is vulnerable to failure since the clamping force is concentrated on the nose of the front portion. Furthermore, the clamping force is weak.